Electrical steel sheets are broadly divided into non-oriented electrical steel sheets and grain-oriented electrical steel sheets. Non-oriented electrical steel sheets are used mainly in the cores of rotating machines and the like and grain-oriented electrical steel sheets are used mainly in the cores of power transformers. Both are generally required to be materials with low iron loss in order to reduce energy loss. Owing to the need for a surface insulating film, they are made into products after being coated with an insulating coating. Since grain-oriented electrical steel sheets almost always contain Si, they are also referred to as grain-oriented silicon steel sheets.
In an grain-oriented electrical steel sheet whose crystal orientation is aligned in the rolling direction, i.e., a grain-oriented electrical steel sheet, the iron loss can be decreased by imparting tension to the steel sheet. For imparting tension to the steel sheet, it is effective to form at a high temperature a film composed of a material whose coefficient of thermal expansion is smaller than that of the steel sheet. This utilizes the thermal stress generated by the thermal expansion coefficient differential between the steel sheet and the film. On the surfaces of ordinary grain-oriented electrical steel sheet there is present a film composed mainly of forsterite formed during finish annealing (hereinafter called the "finish annealing film") by reaction between an oxide film composed mainly of SiO.sub.2 produced at the decarburization annealing step and the MgO commonly used as an annealing separation agent. This finish annealing film effectively reduces iron loss by imparting a large tension to the steel sheet.
Further, JP-A-(unexamined published Japanese patent application) 48-39338 teaches an insulating film obtained by coating the surfaces of a steel sheet with a coating solution composed mainly of colloidal silica and phosphate, followed by baking. This insulating film effectively reduces iron loss by imparting a large tension. The ordinary method of producing a grain-oriented electrical steel sheet is therefore to impart an insulating film without removing the film produced in the finish annealing step.
Attempts have been made to increase the tension to the steel sheet by an insulating film. For instance, JP-A-6-306628 teaches an Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 -system crystalline film obtained by baking on a coating solution composed mainly of alumina sol and boric acid, which, for the same film thickness, enables a film tension to be secured that is 1.5-2 times that secured when a coating solution composed mainly of colloidal silica and phosphate is baked on.
On the other hand, it was recently learned that a disorderly interface structure between the finish annealing film and the steel matrix to some degree cancels the effect of film tension on the iron loss.
Thus, as disclosed in JP-A-49-96920 and JP-A-4-131326, for example, technologies have been developed that attempt to further reduce iron loss by use of a mechanical method such as polishing or grinding, or of a chemical method such as pickling, to remove the finish annealing film occurring in the finish annealing step, or by preventing formation of a finish annealing film in the finish annealing, thereby achieving a state with substantially no finish annealing film or a near mirror state, and then freshly imparting a tension film.
Avoiding formation of a finish annealing film has other advantages aside from lowering iron loss. The film composed mainly of forsterite formed by finish annealing is hard and the cuttability of the steel sheet is poor. As taught by JP-A-64-62476, therefore, it has been proposed to include an additive in the annealing separation agent utilized in finish annealing so as to hinder formation of a finish annealing film and is thereafter impart an insulating film.
However, the adhesion property of the insulating film, although considerable when the insulating film is formed on a finish annealing film, is generally inferior when no finish annealing film is substantially present, such as in a case where the finish annealing film is removed or formation of a finish annealing film is deliberately avoided in the finish annealing step. In particular, tight film adhesion is not obtained at all when the insulating film has tension-imparting property. Even an insulating film without tension-imparting property will loose its tight adhesion property if applied thickly to secure high insulating property.
However, the following is possible. It is generally considered difficult to form a strong chemical bonding force between an oxide and a metal. In insulating film formation, the baking is generally conducted by continuous annealing and the annealing time has to be set on the order of several minutes in order to achieve reasonable productivity. This generally makes it difficult to obtain a chemical bonding force adequate for obtaining tight adhesion between an insulating film composed of oxide and the steel matrix. In the case of a finish annealing film, on the other hand, several tens of hours can be used for film formation because finish annealing is batch annealing. Even though the reaction that forms the bonding force between the film and the steel matrix proceeds slowly, good adhesion can be obtained in the end owing to the long annealing time. As both the finish annealing film and the insulating film are oxides, tight mutual adhesion can be easily obtained even if the insulating film formation time is short.
Therefore, when attempting to eliminate the finish annealing film and to form the insulating film directly on the steel matrix, a technology enabling tight adhesion between the insulating film and the matrix is necessary in order to reduce the iron loss value of a grain-oriented electrical steel sheet to the absolute minimum. Even when a finish annealing film is present, moreover, the adhesion of the insulating film becomes unstable when the finish annealing film is thin or the finish annealing film is absent.
Regarding this problem, the inventors of JP-A-6-184762 proposed a method for improving the adhesion property of a tension-imparting type insulating film with respect to a grain-oriented electrical steel sheet with no finish annealing film. Specifically, this is a method of forming a SiO.sub.2 film having good adhesion property with the steel matrix before insulating film formation. As explicit SiO.sub.2 film forming methods, JP-A-6-184762 set outs a method of forming a SiO.sub.2 film by annealing in a weak reducing atmosphere to selectively oxidize Si inherently contained in a silicon steel sheet and a method that uses CVD, PVD or other dry coating. In the case of annealing in a reducing atmosphere, however, annealing equipment enabling atmosphere control is newly required, while in the case of dry coating, vacuum deposition equipment is necessary. These two methods therefore have a problem regarding processing cost.